1. Field of the Invention
This invention relates to the field of video encoding, and more particularly relates to motion estimation in video encoding.
2. Description of the Related Art
To represent a picture image digitally, the image area is described as an array of pixels. A digital number describes the color, luminance and chrominance of each pixel. Pixel color information consists of three digital values: one digital value for red, one for green, and one for blue. Thus, a fairly large volume of data is required to describe one pixel. Accordingly, exceptionally large data files are required for complete picture images.
In full motion video, not only are large blocks of data required to describe each picture image, but a new image or frame must be presented to the viewer at approximately thirty new images per second to create the illusion of motion. Moving these large streams of video data across digital networks or phone lines is infeasible given currently available bandwidth.
Data compression is a technique for reducing the number of bits required to represent a given image. Data compression techniques utilize either a shorthand notation to signal a repetitive string of bits or omit data bits from the transmitted message. The latter form of compression is called “lossy” compression and capitalizes upon the ability of the human mind to provide the omitted data. In motion video, much of the picture data remains constant from frame to frame. Therefore, the video data may be compressed by first describing a reference frame and describing subsequent frames in terms of the change from the reference frame.
Several international standards for the compression of digital video signals have emerged and more are currently under development. These standards apply to algorithms for the transmission and storage of compressed digital video in a variety of applications, including: video-telephony and tele-conferencing; high quality digital television transmission on coaxial and fiber-optic networks as well as broadcast terrestrially and over direct broadcast satellites; and in interactive multimedia products on CD-ROM, Digital Audio Tape, and Winchester disk drives.
Several of these standards involve algorithms based on a common core of compression techniques, e.g., the CCITT (Consultative Committee on International Telegraphy and Telephony) Recommendation H.120, the CCITT Recommendations H.261 and H.263, and the ISO/IEC MPEG-1, MPEG-2, and MPEG-4 standards. The MPEG algorithms were developed by the Moving Picture Experts Group (MPEG), as part of a joint technical committee of the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). The MPEG standards describe a compressed representation of video and associated audio signals. The standard specifies the syntax of the compressed bit stream and the method of decoding, but leaves considerable latitude for novelty and variety in the algorithm employed in the encoder.
Motion compensation is commonly utilized by video encoders in signal processing techniques that compress successive frames of digital video data for transmission via a communication medium of limited bandwidth, or for storing in a storage medium having limited storage capacity. Motion compensated video compression systems such as the ISO/ITU standards of MPEG and H.261/3 use block-based motion estimation that compares a given block of one frame to a block of another frame. Blocks are matched by determining a comparison measurement between any given pair of blocks. A comparison measurement corresponds to some form of a degree of “difference” between the two blocks. If the comparison measurement is below a predetermined threshold, the blocks may be considered to be similar enough that a block match is indicated. If so, the block in the previous video frame may be utilized and only a motion vector is required to indicate the new position of the block in the current video frame. Such motion vectors can be represented with fewer bits than the pixels that comprise the block, and fewer bits need to be transmitted (or stored) in order to recreate the block. A compression technique known as transform coding is often used to generate a bitstream to be encoded as further described hereinbelow.
Motion compensation and encoding motion comensated video are of the most computationally intensive tasks that a video encoder performs. The objective of the encoder is to produce an encoded image represented in a bitstream that provides the best visual quality for the rate of data transfer, also referred to as bit-rate, allowed by the video coding standards.